The present invention relates to an optical pick-up device suitable for use with an apparatus such as an optical disc player, a compact disc player or an optical video disc player.
FIG. 1 shows a block diagram of a conventional optical pick-up device used in an optical disc system. Rays of divergent light, i.e., laser beams, emerging from a light source 1 such as a semiconductor laser or the like are reflected by a surface of a parallel plane plate 2 and thereafter strike on objective lens 3. The objective lens converges the light incident thereto and irradiates a disc 4 (recording medium) with the converged light.
The rays of divergent light reflected from the disc 4 are condensed by the objective lens 3. The rays then pass through the plane 2 and are converged on a photo detector 5. The plate 2, e.g., an optical member formed with a glass plate, is disposed in the middle of a light path leading to the photo detector 5, and hence astigmatism is created in the light incident upon the photo detector.
The photo detector 5 is, as depicted in FIGS. 2(aa)-2(d) divided into four regions D.sub.1 to D.sub.4 by first and second rectilinear lines 51 and 52, respectively. The line 51 is drawn parallel to a track on the disc 4 and the line 52 is perpendicular to the line 51. When the light is properly focused on the disc 4, the spot light incident on the photo detector 5 assumes a substantially circular configuration, as illustrated in FIG. 2(b). In contrast, if the disc 4 is spaced farther away from the objective lens 3 or closer thereto than the focal distance, the spot light assumes a substantially vertically or laterally elongated elliptical shape as shown in FIGS. 2(a) and 2(c), respectively. This phenomenon enables focussing and tracking errors to be corrected using a so-called astigmatism method and a so-called push-pull method. In the astigmatism method, a focus error signal is generated based on a difference between a first sum of outputs of the regions D.sub.1 and D.sub.3 and a second sum of outputs of the regions D.sub.2 and D.sub.4. In the push-pull method, a tracking error signal can also be produced based on a difference between a third sum of outputs of the regions D.sub.1 and D.sub.2 and a fourth sum of outputs of the regions D.sub.3 and D.sub.4.
The above description and methods are valid when based on the assumption that ideally only an astigmatism is created by the plate 2. However, the plate 2 also causes a coma aberration. Consequently, the spot light on the photo detector 5 in a well-focused state really assumes a substantially trapezoidal shape (FIG. 4) rather than a circular shape. FIG. 4 illustrates a situation where the light from the spot light which is incident on the disc 4 (at "an information detecting point") is projected (light beam tracking) on the photo detector 5 through the objective lens 3 and the plate 2 which cause a coma aberration and an astigmatism. The light with which the disc 4 is irradiated is, as illustrated in FIG. 3, diffracted by a track (e.g. a pit thereof) on the disk, while the objective lens 3 serves to condense the Oth-dimensional diffraction light and positive/negative (.+-.) dimensional diffraction light (created by the plate 2) in FIG. 4.
An exemplary distribution of light on the photo detector 5 in the case of densely arranged tracks (narrow track pitch) is depicted in FIG. 5(a). For sparsely arranged tracks (wide pitch), exemplary distributions are shown in FIGS. 5(b)-5(d), in order of increasing track pitch. The first curved line 61 has (-) primary diffraction light and Oth-dimensional light to its left, where the line 61 is concave. The second curved line 62 has (+) primary diffraction light and Oth-dimensional light to its right, where the line 62 is concave. In the area of FIG. 4 between the convex portions of the curved lines 61 and 62, there is Oth-dimensional light. It can be observed from the Figures that the distribution curve of the (.+-.) primary diffraction light is not symmetric with respect to the rectilinear dividing line 51 and overlapped the line 51 as the track pitch increases (FIGS. 5(b)-5(d)). To aid in understanding the diffraction pattern on the photo detectors, FIGS. 6(a)-6(d) respectively show the distribution curve and the dividing lines 51, 52 on the photo detector 5 in FIGS. 5(a)-5(d) projected from the photo detector onto the objective lens 3 through the plate 2.
An example of tracking control using the diffraction pattern of FIGS. 5(b) and 6(b) will now be explained. In FIGS. 5(b) and 6(b, a quantity (D.sub.1 +D.sub.2) of the (-) primary diffraction light in the regions D.sub.1 and D.sub.2 is equal to a quantity (D.sub.3 l +D.sub.4) of the (+) primary diffraction light in the regions D.sub.3 and D.sub.4. The quantity of Oth-dimensional diffraction light in the regions D.sub.1 and D.sub.2 (D.sub.1 +D.sub.2) is greater than that in the regions D.sub.3 and D.sub.4 (D.sub.3 +D.sub.4). As a result, even when adequate tracking of the spot light on the disc 4 is performed (that is, when the quantities of rays of .+-. primary diffraction light are equal to each other), the Oth-dimensional diffraction light and the (.+-.) primary diffraction light are synthesized, whereby the generated tracking error signal does not become zero (i.e. (D.sub.1 +D.sub.2)-(D.sub.3 +D.sub.4) .noteq.0). Therefore, if the objective lens 3 is tracking-controlled using the tracking error signal, the spot light (information detecting point) will respond to the tracking error signal by moving farther away from the track on which it is intended to be directed. To compensate for this problem, it has been proposed to add an offset amout to a sum of one group of outputs (e.g., D.sub.3 l +D.sub.4). An amount of the offset is, however, dependent on the track pitch and a reflection rate of the disc 4. Hence, it is difficult to effect an adequate offset adjustment if there is some variance in the track pitch and/or reflection rate.
When searching a predetermined track, the spot light (the information detecting point) traverses the track. At this time, an imbalance takes place in the (.+-.) primary diffraction light: one (e.g. (+)) is bright, whereas the other (e.g. (-)) is dark. The distribution of (-) primary diffraction light in the region D.sub.1 is wider than in the region D.sub.2. Similarly, the distribution of (+) primary diffraction light in the region D.sub.4 is wider than in the region D.sub.3. Consequently, the (-) primary diffraction light becomes dark, whereas the (+) primary diffraction light becomes bright. In this case, the sum of outputs of the regions D.sub.1 and D.sub.3 decreases, while the sum of outputs of the regions D.sub.2 and D.sub.4 increases. Therefore, the information detecting point traverses the track. It follows that an offset component is produced in the focus error signal.
As explained above, it is difficult to properly determine the tracking error signal and the focus error signal because of the comatic aberration in the conventional devices. To cope with this difficulty, as illustrated in FIG. 7, the conventional device is arranged such that another parallel plane plate 6 (or a cylindrical lens) is disposed facing in an opposite direction to the plate 2 to correct the comatic aberration thereof. This arrangement, however, requires increasing the number of parts, consequently increasing the cost, complexity and size of the device.